Electric components are normally mounted on a printed circuit board by bonding electrodes of the electric components to the printed circuit board by means of soldering for establishing electrical connection. Conventionally, lead wires of electric components are inserted in through holes of a printed circuit board for soldering. However, because of the miniaturization of recent electrical devices, electric components without lead wires, such as SMP (surface mount parts) have become widely used which are mounted on a double-sided reflow (molten) substrate. Thus, the recent soldering method is different from the conventional method in which the lead wires are inserted into the through holes and then soldered.
Nevertheless, the conventional components are still necessary today, resulting in a mixture of the conventional components with lead wires and recent components without lead wires on a printed circuit board. For mounting these components on a printed circuit board, after the lead wires being inserted in the through holes, the printed circuit board is put through a flow soldering bath for soldering the part where the lead wires are inserted. Such a printed circuit board is called a flow substrate (hereafter referred to as “substrate”).
When mounting such electrical components having traditional lead wires to the substrate, a soldering apparatus with a jet nozzle which is able to apply the solder to a specified part (local area) of the substrate is used in order to increase the work efficiency. FIG. 3 is a schematic perspective view showing the essential portion (particularly, a jet nozzle portion) of such a jet nozzle type soldering apparatus.
In the jet nozzle type soldering apparatus (hereafter “soldering apparatus”) M, molten solder h is contained in a soldering bath b and a part of which is circulated in the soldering apparatus by a circulation means (not shown). More specifically, the molten solder h is filled into a tubular body 20 through pipes 40 positioned at the bottom of a rectangular tubular jet nozzle 10 mounted on a base 30. The tubular body 20 of the jet nozzle 10 has an upper opening with an opening rim 20e. The molten solder h flows out in the direction as shown by an arrows a through cutouts 20m formed at the opening rim 20e at the top of tubular body 20, returning to the soldering bath b.
Because an oxide film will be formed on its surface if the surface of the molten solder h stands still, in the soldering apparatus M, the molten solder h has to be overflowed from the cutouts 20m of the jet nozzle 10 to constantly circulate the molten solder h so as not to grow the oxide film.
In the above noted conventional jet stream type soldering apparatus, as shown in FIG. 4, when a space W between two electrical components d on a substrate p is wide enough as shown in FIG. 4A, the overflowing portion f of the molten solder h (hereafter also referred to as “solder flow”) does not come in contact with the adjoining electrical component. Recently, however, demands for a smaller and even more highly dense substrate are increasing, requiring further decrease in the pitch between the electrical components. Thus, a space N between the electric components has become smaller as shown in FIG. 4B.
Further, in consideration of the environment issues, lead-free solder (tin-silver-copper) has been in use instead of the conventional lead containing solder (eutectic solder of tin 6:lead 4). The lead-free solder, in comparison to the conventional lead containing solder (melting temperature of about 183° C.) has higher solidus (melting temperature of about 218° C.), which easily solidifies within the nozzle if the amount of circulation is small. Therefore, it is necessary to increase the amount of circulation of solder flow by increasing the amount of overflows of the molten solder from the cutouts 20m. 
However, this means that the amount of molten solder flowing in the transversal direction from the cutouts 20m will also increase. Thus, in the case of having two electrical components close together as in FIG. 4B, the molten solder may contact with the lead wire 1 of the adjoining electrical component d, causing problems that are difficult to resolve. Since the amount of the solder flow is increased, a solution of this problem by increasing the number of cutouts 20m can be conceivable. However, because the space in the jet nozzle is limited due to the closeness of the electrical components, this solution is not practical to resolve the problem.
Moreover, in the method of overflowing the molten solder from the opening rim 20e noted above, flux that has been applied to the local area of the substrate in advance will be washed away by the surface flow of the molten solder, causing soldering errors.